CHATTAHOOCHEE HILLS, GEORGIA—On a recent drizzly morning, I tromped through pine needles and mud past a couple of glowering llamas and through a wildflower meadow until reaching Selborne, one of three neighborhoods in Serenbe, a 1,000-acre intentional community southwest of Atlanta. Around 600 people live in Serenbe’s 350 homes, and plans are afoot for many more.

The community’s founder and developer, Steve Nygren, has built Serenbe over the past 15 years. At 71, he easily outpaced me on our two-…